The present invention relates to single-walled carbon nanotubes (SWCNTs), which may be used for thermoelectric (TE) power generation. The development of inexpensive and efficient TE materials offers the prospect of converting waste heat into pollution-free electricity in standalone power generation systems, cogeneration architectures (e.g., coupled to a photovoltaic module), and/or cooling systems (e.g., microprocessor cooling). It is desirable for efficient TE materials to be good at conducting electricity but not heat, so that a thermal gradient may be maintained to produce the TE effect. However, this is challenging for most material systems, because the electrical and thermal conductivities are typically related to each other via the charge carrier density, such that the thermal conductivity increases as the electrical conductivity increases. Decoupling the electrical and thermal conductivities has been achieved in some inorganic semiconductors (ISCs), such as bismuth telluride (Bi2Te3), although further improvements in these materials are likely to require the development of complex and/or nanoscale structures. Complex fabrication strategies, combined with material cost, scarcity, toxicity, and disposal, may significantly limit the potential for large-scale deployment of TE devices based on such materials.
The size-tunable physical properties of solution-phase processable nanomaterials may enable diverse strategies for energy harvesting/storage and inexpensive, bottom-up approaches for fabricating devices with unique form and function (e.g., flexible, lightweight, and/or wearable). Nanostructuring of bulk ISCs has shown particular promise for improving TE energy conversion devices, which convert thermal energy from waste and natural heat sources into electricity, due to the beneficial formation of nanoscale interfaces. However, the best-performing ISCs are incompatible with applications that require the TE generator to adopt irregular, or even flexible, form factors.
In contrast, nanostructured organic semiconductors (OSCs), including SWCNTs, offer a number of intriguing technological characteristics for TE applications, such as earth-abundant raw materials, low-cost deposition, and flexible form factors. Despite their promising electronic properties, SWCNTs have received little attention in the context of TE energy conversion, although several studies have focused on the use of SWCNTs as inclusions in composite materials based on conducting polymers. Two recent studies demonstrated higher thermopowers for films enriched in semiconducting (s-SWCNT) species than those containing significant fractions of metallic (m-SWCNT) species. Beyond these results, little has been known about the detailed dependence of the TE power factor and the thermal conductivity on the SWCNT diameter, electronic structure, and carrier density. Although large thermal conductivities (κ>1,000 W m−1 K−1) have been observed for individual SWCNTs, much lower values (κ<35 W m−1 K−1) have been obtained for mats of nanotube ropes or bundles.